Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
3.0
Nickel Metal Hydride (NiMH)
3.1
NiMH Principles of Operation
Section 3
The principles in which NiMH cells operate are based on their ability to absorb, release, and transport (move)
hydrogen between the electrodes within the cell. The following sections will discuss the chemical reactions
occurring within the cell when charged and discharged and the adverse effects of overcharge and overdischarge
conditions.
The success of the NiMH battery technology comes from the rare earth, hydrogen-absorbing alloys (commonly
known as Misch metals) used in the negative electrode. These metal alloys contribute to the high energy
density of the NiMH negative electrode that results in an increase in the volume available for the positive
electrode. This is the primary reason for the higher capacity and longer service life of NiMH batteries over
competing secondary batteries.
3.2
Charging Chemical Reaction
When a NiMH cell is charged, the positive electrode releases hydrogen into the electrolyte. The hydrogen in turn
is absorbed and stored in the negative electrode. The reaction begins when the nickel hydroxide (Ni(OH)2) in
the positive electrode and hydroxide (OH¯) from the electrolyte combine. This produces nickel oxyhydroxide
(NiOOH) within the positive electrode, water (H20) in the electrolyte, and one free electron (e ¯). At the negative
electrode the metal alloy (M) in the negative electrode, water (H20) from the electrolyte, and an electron (e ¯)
react to produce metal hydride (MH) in the negative electrode and hydroxide (OH¯) in the electrolyte. See
Figure 3.2 Chemical Equations and Figure 3.3 Transport Diagram.
Because heat is generated as a part of the overall chemical reaction during the charge of a NiMH cell, the
charging reaction described above is exothermic. As a cell is charged, the generation of heat may not
accumulate if it is effectively dissipated. Extreme elevated temperatures may be experienced if a cell is
excessively overcharged. See Section 3.4 Overcharge and 3.5 Overdischarge.
Figure 3.2 Chemical Equations
Positive Electrode:
-
charge
Ni(OH)2 + OH
NiOOH
-
+ H2O + e
discharge
Negative Electrode:
-
charge
M + H2O + e
-
MH + OH
discharge
charge
Overall Reaction:
Ni(OH)2 + M
NiOOH + MH
discharge
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
Figure 3.3 Transport Diagram
I
CHARGER
e-
e-
–
+
H2O
+
H
OH
OH
-
OH
OH
Ni(OH)2
-
-
+
Ni(OO
+
H
H
H2O
POSITIVE ELECTRODE (Ni)
NEGATIVE ELECTRODE (MH)
H2O
+
H
H2O
ELECTROLYTE
–
+
e-
Metal Hydride
Hydrogen
3.3
I
DISCHARGE
e-
Discharge Chemical Reaction
When a NiMH cell is discharged, the chemical reactions are the reverse of what occurs when charged.
Hydrogen stored in the metal alloy of the negative electrode is released into the electrolyte to form water. This
water then releases a hydrogen ion that is absorbed into the positive electrode to form nickel hydroxide. See
Figure 3.2 Chemical Equations and Figure 3.3 Transport Diagram.
For NiMH cells, the process of moving or transporting hydrogen from the negative electrode to the positive
electrode absorbs heat and is therefore endothermic. Heat continues to be absorbed until the cell reaches a
state of over discharge, where a secondary reaction occurs within the cell resulting in a rise in temperature. See
Section 3.5 Over discharge.
3.4
Overcharge
Nickel Metal Hydride cells are designed with an oxygen-recombination mechanism that slows the buildup of
pressure caused by overcharging. The overcharging of a cell occurs after the positive electrode 1) no longer has
any nickel hydroxide to react with the hydroxide from the electrolyte, and 2) begins to evolve oxygen. The
oxygen diffuses through the separator where the negative electrode recombines the oxygen with stored
hydrogen to form excess water in the electrolyte. If this oxygen-recombination occurs at a slower rate than the
rate at which oxygen is evolved from the positive electrode, the result is in a buildup of excess oxygen (gas)
resulting in an increase in pressure inside the cell. To protect against the first stages of overcharge, NiMH cells
are constructed with the negative electrode having a capacity (or active material) greater than the positive
electrode. This helps to slow the buildup of pressure by having more active material available in the negative
electrode to effectively recombine the evolved oxygen. See Figure 3.4, Useable Capacity Diagram.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
Excessive overcharging of a NiMH cell can result in permanent loss in capacity and cycle life. If a cell is
overcharged to the point at which pressure begins to build up, elevated temperatures are experienced and can
cause the separator to lose electrolyte. The loss of electrolyte within the separator (or “separator dry out”)
inhibits the proper transport of hydrogen to and from the electrodes. Furthermore, if a cell is severely
overcharged and excessive amounts of oxygen (gas) are evolved, the pressure may be released through the
safety vent in the positive terminal. This removes elements from within the cell needed for proper function. To
protect against the damaging effects of overcharging, proper charge terminations must be used. See Section
3.8.2 NiMH Charge Termination.
Figure 3.4 Useable Capacity Diagram
NEGATIVE ELECTRODE
OVERCHARGE
PROTECTION
USEABLE CAPACITY
OVERDISCHARGE
PROTECTION
POSITIVE ELECTRODE
3.5
Over Discharge
There are two phases to the over discharging of a NiMH cell. The first phase involves the active material of the
positive electrode becoming fully depleted and the generation of hydrogen gas begins. Since the negative
electrode has more active material (metal hydride), it has the ability to absorb some of the hydrogen gas evolved
by the positive electrode. Any hydrogen not absorbed by the negative electrode begins to build up in the cell
generating pressure. The second phase begins when the entire negative electrode is fully depleted of active
material. Once both electrodes are fully depleted, the negative electrode absorbs oxygen contributing to the
loss of useable capacity.
Extreme over discharge of a NiMH cell results in excessive gassing of the electrodes resulting in permanent
damage in two forms. First, the negative electrode is reduced in storage capacity when oxygen permanently
occupies a hydrogen storage site, and second, excess hydrogen is released through the safety vent reducing
the amount of hydrogen inside the cell. To protect against the damaging effects of over discharging, proper end
of discharge terminations must be used. See Figure 3.7.4 NiMH Low Voltage Disconnect or Voltage Cutoff.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
3.6
Section 3
Rate Capability
The maximum rate a cell is able to achieve for a short burst is dependent on the cell construction, temperature
and manner in which it is assembled into a pack. This will be clearer after looking at Section 3.7.1 NiMH
Capacity.
3.7
NiMH Discharge Characteristics
The discharge characteristics of NiMH batteries (both cells and battery packs) depend on many factors. These
factors include capacity, voltage, discharge rate, discharge termination (or voltage cutoff), matching (of cells
within a battery pack), internal resistance, and temperature.
3.7.1 NiMH Capacity
Engineers and designers are usually most interested in how long a battery will supply the current
needed to run a piece of equipment or a device. The length of time is directly proportional to the
capacity of the battery and discharge rate. Defined as C, capacity is the electric current content of a
battery expressed in ampere-hours (Ah) or milli-ampere-hours (mAh). The capacity of a battery is
determined by discharging the battery at a known constant current until a predetermined end voltage is
reached. The amount of time it takes to discharge a battery to the end voltage multiplied by the rate of
current at which the battery was discharged is the rated capacity of the battery. Therefore, a battery
would be rated at 1500 mAh if it was discharged at a rate of 150 mA to an end voltage of 1.0 volt per
cell and it discharged for 10 hours.
For clarification, the rate of current (charge or discharge) that is applied to a battery is often defined in
terms of the rated capacity C of a battery. For example, a battery rated at 1500 mAh that is discharged
at a rate of C/2 (or 0.5C) will have 750 mAh discharged from the battery per hour. Thus, a discharge
rate of C/2 of a 1500 mAh battery is 750 mA, but this does not mean the battery will last for 2 hours!
One of the biggest misconceptions regarding NiMH cells is that rated capacity is the capacity that will be
received by the user. This would only be true if the user charged and discharged at the same rates of
current at which the cell was graded.
Rated capacity has been defined by the International Electrotechnical Commission (IEC) in document
#61436.1.3.4, as a charge at a rate of 0.1C for a period of 16 hours. This is followed by a discharge of
0.2C to a voltage of 1.00V per cell. However this number sometimes can be convoluted by Maximum,
Typical, and Minimum cell ratings. For example, a lot of 1000 cells might range from 1000 mAh to 850
mAh. The maximum capacity would then be 1000 even though only a small number of the cells attain
this capacity. The nominal capacity would be 900 mAh, and the majority of the cells tested would attain
this capacity. All those cells must conform to the minimum of 850 mAh. We have found that to be
considered with other manufacturers, we must also conform to this cell rating procedure
Results derived when testing NiMH technology battery packs for capacity can change dramatically with
variation in:
• Temperature
• Charge Rate
• Discharge rate
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
•
Section 3
Number of Cells in the Pack / Increase in Voltage Cutoff per Cell
All of these conditions must be taken into to account when a comparison is being made from pack to
pack, and cell-to-cell.
Polarizations
When a NiMH technology cell is subjected to a flow of current a chemical change occurs within
the cell. The obstacles to current flow are known as polarizations. The polarizations are:
!
!
!
1
Ohmic- Ohmic polarization is the internal impedance of the cell against current flow. The
impedance has a corresponding voltage drop, which can be seen in the voltage profile of
any NiMH cell. During charge current this voltage is added to the overall voltage of the cell.
However during discharge this voltage is subtracted from the overall voltage of the cell. The
magnitude of the voltage drop will be directly proportional to overall impedance internal to
the cell and the rate of current (charge/discharge) the cell is subjected to.
If a cell has higher impedance, the drop will be greater.
As the current applied to the cell is increased the voltage drop will increase. This is critical
when a predetermined termination voltage is set. If a cell is discharged at a high enough
current it can instantly force the cell voltage below 1.00V, even though there is almost
100% capacity remaining.
!
Concentration- Concentration polarization is directly proportional to the surface area of the
anode and cathode of the cell. The greater the surface area of these active plates, the
greater the reduction in this polarization. This is determined during the engineering and
manufacturing of the cell itself, and little can be done after that.
!
Activation- Activation polarization is the amount of energy expended to cause the chemical
reaction. Nothing is 100% efficient, so any chemical change expends energy. At a
molecular level temperature has a great effect on the amount of energy it takes to make a
reaction transpire. Generally all rating is performed at 20-25°C, as the temperature
increases, or decreases the amount of energy on discharge will change.
!
Knowing what affects the NiMH chemistry, we can fashion testing to isolate any variable
that may cause inconsistent results.
!
For example, a cell is charged at a 0.5 C with a –dV termination. When capacity testing is
performed at a 0.5C discharge expect to see between 5-7% reduction in cell capacity over
the IEC rating. This is explained first by the Ohmic polarization on discharge, and secondly
due to the variation in charged energy using a –dV charge regime.
!
In order for a –dV to be seen on a NiMH cell it must be subjected to a small margin of
overcharge. This overcharge condition forces the voltage depression. It occurs when all of
the active material in the cell is chemically reacted, and oxygen evolves and then diffuses
into the negative electrode. The negative electrode is designed to be larger than the
positive so it accepts a portion of this oxygen diffusion, however pressure is still generated
within the cell, which generates heat. The heat then causes the cell voltage to drop. A direct
comparison of the energy input during charge and output during discharge cannot be made
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
because a portion of the input energy was wasted on oxygen evolution and was never
recombined. In order to derive a % of rated capacity, we must calculate the energy received
during discharge of our test, with that cell’s IEC rated capacity
3.7.2 NiMH Voltage
The discharge voltage profile of a NiMH battery is considered “flat” (see Figure 3.7.2 C/10 Discharge
Profile @ 25°C) and varies with the rate of discharge and temperature. As a fully charged battery is
discharged the voltage begins at about 1.5 volts followed by a sharp drop to around 1.3 volts. The
voltage remains between 1.3 to 1.2 volts for about 75% of the profile until a second sudden drop in
voltage occurs as the useful capacity of the battery begins to deplete. At this point is where the
discharge current (or load) is terminated at a safe voltage level (see Section 5.4 Low Voltage
Disconnect or Voltage Cutoff). With elevated discharge rates, the entire discharge profile is lowered by
losses in ohmic polarizations (internal resistance). At high temperatures, the discharge profile is raised
by an increase in potential (voltage) between the electrodes. At temperatures below 10°C (50°F),
concentration polarization significantly lowers the voltage and the useable capacity. This is caused by
an increase in energy required to transport molecules within the battery. See Section 2 Principles of
Operation and Construction.
The industry standard for the rated voltage of a NiMH cell is 1.2 volts. This value is the nominal voltage
of a cell that is discharged at a rate of C/10 at a temperature of 25°C (77°F) to an end voltage of 1.0
volt. This industry standard is used primarily to call out the rated voltage of battery packs. For example,
a battery pack made of three cells in series would be rated as a 3.6-volt battery pack. Figure 3.7.2, C/10
Discharge Profile @ 25°C, shows that the nominal voltage of a Quest® NiMH cell is just above 1.2 volts.
For technical applications and calculations, the nominal voltage of a battery pack provides a useful
approximation of the average voltage throughout discharge. The nominal voltage can be simply
calculated after the battery has been discharged. To calculate the nominal voltage, divide the batteries
energy [watt-hours (Wh)], by the capacity, [ampere-hours (Ah)]. This calculation proves beneficial when
a battery is discharged at high temperatures, since nominal voltage will increase under these conditions.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
Figure 3.7.2 C/10 Discharge Profile @ 25°C
1.5
Voltage (volts)
1.4
1.3
1.2
1.1
1
0
2
4
6
8
10
12
Time (hours)
3.7.3 NiMH Discharge Rate
The delivered capacity and nominal voltage of a battery are dependent on the rate of current at which a
battery is discharged. For NiMH batteries, there is no significant affect on capacity and voltage for
discharge rates below 1C. A reduction in the nominal voltage occurs for discharge rates between 1C
and 3C for all the NiMH cell sizes with the exception of the high rate series of cells.
3.7.4 NiMH Low Voltage Disconnect or Voltage Cutoff
To prevent potential irreparable harm to a battery due to polarity reversal of one or more cells during
discharge, the load (discharge current) must be terminated prior to the battery being completely
discharged. Damage can be avoided by terminating the discharge at a point where essentially all
capacity has been obtained from the battery, but a safe voltage level remains. Removing the load in
this manner is referred to as Low Voltage Disconnect or Voltage Cutoff. Capacity of a battery is slightly
dependent on the Voltage Cutoff used at the end of discharge. Continuing the discharge to lower end
voltages can slightly increase the delivered capacity, yet if the end voltage is set below the
recommended Voltage Cutoff the cycle life of the battery will be decreased. The following table gives the
Voltage Cutoff recommendations for discharge rates of less than 1C:
Figure 3.7.4 Voltage Cutoff Schedule
1
Number of Cells in Series
Low Voltage Disconnect/Voltage Cutoff
1 to 6
7 to 12
1 volt per cell
[(MPV-150mV)(n-1)]-200mv
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
Where MPV is the single cell midpoint voltage at the given discharge rate (typically 1.3) and n is the
number of cells in the pack. For discharge rates greater than 1C the MPV will decrease.
3.7.5 NiMH Matching
Matching refers to the grouping of individual NiMH cells with similar capacities to be used within a
battery pack. Typically, the matching of the cells in a battery pack is within 2%. Matching eliminates the
potential of reversing the polarity of one or more cells in a battery pack due to the capacity range of the
combined cells being too great. Matching becomes more critical as the number of cells in the battery
pack increases. This is due to the potential of one cell having a capacity significantly lower than the
average capacity of the other remaining cells. As a result, the lowest capacity cell has the potential to
reverse polarity while the other cells remain at safe voltage levels before reaching the voltage cutoff. If
a battery pack has one or more cells reversed before the Voltage Cutoff is reached, the performance
and cycle life will be reduced.
3.7.6 NiMH Internal Resistance
The internal resistance of NiMH cells varies depending on cell size, construction, and chemistry.
Different materials are used in various NiMH cells to achieve desired performance characteristics. The
selection of these materials also affects the internal resistance of the cell. Since NiMH cells of various
sizes, construction, and chemistry are different, there is no one value of internal resistance that can be
defined as a standard. The internal resistance for each cell is measured in mΩ at 1000Hz.
3.7.7 NiMH Temperature
To obtain the optimum capacity and cycle life of a NiMH battery, the recommended range of
temperature when discharging a standard battery is 0°C (32°F) to 40°C (104°F). Due to the
endothermic discharge characteristics of NiMH cells, the discharge performance is increased
moderately at higher temperatures, yet the cycle life will be lowered. At lower temperatures,
performance decreases more significantly due to the cells polarization. See Section 3.7.7. This is
caused by a decrease in transport capabilities (the ability to move ions within the electrodes). For most
NiMH batteries, the following chart shows the typical affect of temperature on capacity at a C/5
discharge rate.
1
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
Figure 3.7.7 Discharge Capacity vs. Temperature
3.8
NiMH Charge Characteristics Overview
The charging, or recharging, of NiMH batteries (both cells and battery packs) is the process of replacing the
energy that has been removed or discharged from the battery. The performance of the battery, as well as the
cycle life, depends on effective charging. The three main criteria for effective charging are:
! Choose the appropriate charge rate
! Select the appropriate charge termination technique
! Control the temperature
3.8.1 NIMH Charge Rates
As the capacity of NiMH cells has increased, so has the demand for faster charging. This leads to
higher charge rates, which requires care to ensure a complete charge while minimizing the potential
damage of overcharging. Slow charging is still a reliable method, but not all batteries can be slow
charged without some type of termination. Some NiMH cell chemistries have higher capacities and are
better suited to the more popular fast charging methods.
3.8.2 NIMH Charge Termination
Properly controlling the charging of a NiMH battery is critical to achieving optimum performance. Charge
control incorporates proper charge termination to prevent overcharging the battery. The overcharging of
a battery refers to the state at which the battery can no longer accept (store) the energy entering the
battery. As a result pressure and temperature builds up within the cell. If a cell is allowed to remain in
the overcharge state, especially at high charge rates, the pressure generated within the cell can be
released through the safety vent located within the positive terminal. This may cause damage to the
battery reducing cycle life and capacity.
To prevent damage occurring to the battery, charge termination is one of the most critical elements to
be applied to any method of charge control. Charge control may utilize one or more of the following
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
charge termination techniques. The three primary techniques of charge termination are time, voltage,
and temperature.
3.8.2.1 Time
Time-based charge control techniques terminate charging of the battery after a predetermined
length of time. This technique should be used when slow charging to avoid excessive
overcharge, and used as a backup secondary termination for all fast charge methods.
3.8.2.2 Voltage
Charge control techniques that are voltage-based are attractive because of the predictable
charge voltage profile of a NiMH battery (see Section 3.8.3 Charge Termination Nomenclature).
The charge voltage profile of a NiMH battery is consistent regardless of the batteries state of
charge. However, the voltage-based charge termination techniques generally occur after a
battery has already reached the overcharge state. In addition, the voltage-based techniques
may not be applicable at rates below C/4 and are prone to false termination due to RF noise. It
is also necessary to include temperature-sensing devices to terminate the charge if the
temperature becomes too high. Such devices include thermostats and PTC resettable fuses.
Peak Voltage Detect (PVD)
The recommended technique for voltage-based charge termination is peak voltage
detect or PVD. This technique involves sensing the drop in voltage after the battery has
reached its peak voltage and becomes overcharged (see Section 3.8.3 Charge
Termination Nomenclature). This technique is recommended because it reduces the
risks of overcharging from that of other voltage charge termination techniques. To
prevent substantial damage to a battery, a maximum drop in voltage of 3 mV per cell
before termination is recommended to limit the amount of overcharge on the battery.
Additionally, the sampling rate of the charger IC is more frequent to increase sensitivity.
Negative Delta V (-∆V)
Negative delta V (-∆V), like PVD, follows the same concept of sensing the drop in
battery voltage after the battery has reached its peak voltage. The difference is the
change or drop in voltage is increased to 3 to 5 mV per cell before the charge is
terminated. This technique allows the battery to be exposed to longer periods of
overcharge and is not normally recommended. See Section 3.8.3 Charge Termination
Nomenclature
3.8.2.3 Temperature
The exothermic nature of NiMH batteries under charge refers to the generation of heat as the
battery is charged especially just before and during overcharge. The temperature-based charge
termination senses this temperature rise and terminates the charge when the battery has
reached a temperature that indicates when full charge is being approached. This type of charge
termination is recommended because of its reliability in sensing overcharge, yet it requires care
1
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
in the selection of set points in the charge circuitry to avoid premature charge termination or
failure to detect the overcharge when the battery is exposed to extreme temperature
environments.
Change in Temperature (∆T)
Change in temperature or ∆T is the technique that measures the difference of the rise
in battery temperature above the starting (ambient) temperature during charge. The
charge is terminated when the rate of change in temperature reaches a predetermined
value. See Section 3.8.3 Charge Termination Nomenclature
Change in Temperature/Change in time (dT/dt)
The recommended technique for temperature-based charge termination for all fastcharging methods is dT/dt (see Section 6.3 Charge Termination Nomenclature). This
technique monitors the change in temperature T verses the change in time t, and is
considered most accurate because it senses the start of overcharge earlier than other
techniques. Baseline dT/dt temperature termination is 1°C per minute, but varies
according to pack design. When using a dT/dt termination, a top-off charge is
suggested in order to fully charge the battery (see Section 3.8.6.5 Top-Off Charge).
Temperature Cut Off (TCO)
Temperature cut off or TCO is a secondary termination required for all fast-charging
methods using dT/dt and/or PVD. This technique is based on the absolute temperature
of the battery and is recommended only as a fail-safe strategy to avoid destructive
heating in case of failure of any or all other charge termination technique(s). See
Section 3.8.3 Charge Termination Nomenclature
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
3.8.3 NIMH Charge Termination Nomenclature
Figure 3.8.3 NiMH Charge Termination Nomenclature
1.80
70
dT/dt - Change in Temperature/Change in Time
PVD - Peak Voltage Detect
TCO - Temperature Cut Off
∆T - Change in Temperature
-∆V - Negative Voltage Detect
1.70
1.65
PTC Resettable Fuse
or Thermostat
- ∆V
65
60
TCO
1.60
55
Voltage (volts)
1.55
50
1.50
PVD
1.45
45
1.40
40
dT
1.35
Voltage
1.30
Temperature
dt
35
Temperature (Celsius)
1.75
dT
∆T
1.25
30
dt
1.20
25
1.15
1.10
20
0
0.5
1
1.5
2
Tim e (hours)
3.8.4 NIMH Temperature and Charge Efficiency
The recommended charging temperature is between 10°C (50°F) and 40°C (104°F). If a NiMH battery
is exposed to high temperatures (above 40°C, 104°F) due to overcharging or external heat sources, the
charge efficiency (increase in stored cell capacity per unit of charge input) will be decreased. In order to
avoid decreased charge efficiency, batteries should have charge control methods applied to limit the
amount of overcharge heat that is generated. In addition, it is critical to not place batteries in close
proximity to other sources of heat or in compartments with limited cooling or ventilation.
At temperatures below 10°C (50°F) charge efficiency will also decrease resulting in an increase in the
amount of time need for charging. Low temperatures inhibit transport capabilities (the ability to move
ions within the electrodes) causing the low charge efficiency (see Section 3.7.1, NiMH Capacity and
Construction; Rate Capability). Charging below 0°C (32°F) is not advisable
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
3.8.5 NIMH Charge Methods
Not all charge methods are recommended for all NiMH cell chemistries, seeing they are not designed
the same. Different materials are used in various NiMH cells to achieve certain desired performance
characteristics. The selection of these materials also affects the charging characteristics of the
batteries. Therefore, any method that could cause problems with some batteries has been noted for
each charge method. See Figure 3.8.5 Charge Method Specifications for recommended charge currents
and charge terminations.
Figure 3.8.5 Charge Method Specifications
Charge
Method
Charge
Current
Slow
Standard
Time
0.02-0.1C
0.1C
0.1-0.2C
Rapid2
0.25-0.5C
Fast2
0.5-1.0C
Maintenance
1
2
Charge
Termination
None1 or Timer
1.
1. Timer
1. Timer, and
2. TCO = 55°C
1. PVD, or dT/dt, or
∆T, and
2. Timer, and
3. TCO = 55°C
1. PVD, or dT/dt, or
∆T, and
2. Timer, and
3. TCO = 55°C
1. None
Comments
Timer rated at 160%C.
Timer set for 16 hours.
Timer rated at 160%C @ 0.1C to 120%C @ 0.2C.
PVD = -∆V of 3-5 mV/cell
dT/dt = ~1°C/1 min rise.
Timer rated at 140%C @ 0.2C to 120%C @ 0.5C.
PVD = -∆V of 3-5 mV/cell
dT/dt = ~1°C/1 min rise.
Timer rated at 125%C.
0.0025-10%C per day at C/128 to C/512 pulse
0.008C
recommended.
Not all batteries can be charged without a termination.
See Rapid/Fast Charging Procedure (Section 3.8.6).
3.8.5.1 Slow Charge
When charge time is not an issue and maximum rechargeable capacity is desired, the slow
charge method is often used. This method uses a charge that is less than 0.1C and for more
than 16 hours (see Figure 3.8.5 Charge Method Specifications). Yet, with the recent
developments of some NiMH cell chemistries to be better suited to faster charging methods,
slow charging is not recommended for all NiMH batteries.
3.8.5.2 Standard Charge
This method can be used for most of the NiMH cell chemistries. The standard charge is a
simple system using a charge rate of 0.1C for 16 hours (see Figure 3.8.5 NiMH Charge
Methods). Since the charge rate is low and the charge is terminated after 16 hours, there is
less risk of overcharge and increased temperatures. The downside to this method is the
inability to detect how much charge a battery has at the time charging begins. Thus, a battery
that is at a 60% depth of charge (DOD), or 40% state of charge (SOC), which is charged using
this method will see the same amount of charge as a fully discharged battery. This leads to the
overcharging of the partially discharged battery before the time termination occurs.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
3.8.5.3 Time Charge
For the faster timed charge method, batteries can typically be charged in 6 to 16 hours. This
method of charging NiMH batteries requires the most attention before selection. Since this
method uses higher rates of current (see Figure 3.8.5 Charge Method Specifications), two
methods of termination are needed: timed and TCO. The later of the two terminations would
require the battery to include a thermistor to detect the temperature during the charge cycle. If
only a timed charged termination was used, the battery may be pushed into overcharge,
especially if a partially discharged battery was charged using this method. For some NiMH cell
chemistries this would significantly deteriorate battery performance.
3.8.5.4 Rapid Charge
The Rapid Charge method is good for applications needing a faster charge time, but the battery
compartment does not allow for good heat dissipation. Rapid charge methods typically charge
in 2.5 to 6 hours using charge rates of 0.25C to 0.5C (see Figure 3.8.5 Charge Method
Specifications). This method of charging uses PVD, - ∆V, dT/dt or ∆T with time backup. For
further charging details see Section 3.8.6 Rapid/Fast Charging Procedure. This system usually
uses a temperature backup to ensure against overcharging.
The advantage of this charge method is the ability to safely charge batteries that are at any
state of charge. In other words, a partially discharged battery can be charged without the risk of
being overcharged. The disadvantage to this type of system is the added complexity and
expense of the charger.
3.8.5.5 Fast Charge
When time is a limited resource and there is good heat dissipation, Fast Charge methods are
the best approach. Fast Charge methods will charge batteries in 2.5 hours or less. Like the
Rapid Charge method, this method has increased charge rates and requires three separate
charge terminations (see Figure 3.8.5 Charge Method Specifications). Some of the higher
capacity NiMH batteries will not handle a constant 1.0C charge rate. Presently, a good rule to
follow is no constant charging above 1.0C or 3.0Amps. For further charging details see Section
3.8.6 Rapid/Fast Charging Procedure.
As with the Rapid Charge, the advantages of the Fast Charge method is the ability to safely
charge batteries that are at any state of charge in a short period of time. The disadvantages
are, again, the added complexity of the charger and expense.
3.8.5.6 Maintenance Charge
Unlike the previous six methods the Maintenance Charge method is not considered a means of
charging a discharged battery to full capacity. Rather, this method is used to counteract the
occurrence of battery self-discharge when the battery is not in use. See Section 3.9 Battery
Storage.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
3.8.6 NIMH Rapid/Fast Charging Procedure
The following procedure outlines the six steps for fast or rapid charging a NiMH battery. These steps will
provide insight into what the charger chip manufacturers have tried to incorporate into their chips.
3.8.6.1 Initialization Charge
Before fast or rapid charging a battery, a trickle charge rate is recommended. Starting with a
pulsed C/10-C/50 trickle charge is good for two reasons. First, to bring up the temperature if
the batteries are cold, and second, to verify there are no problems with the batteries or charging
circuitry.
3.8.6.2 Temperature Measurement
Before fast or rapid charging can begin, the temperature must be between 10°C and 40°C. This
is done as part of the trickle charge step. If a battery has been exposed to lower temperatures,
the battery temperature must be brought up to above 10°C before fast charging can begin.
Furthermore, take note that the dT/dt parameters will be reached at the beginning of a fast
charge on a cold battery, thus resulting in the premature termination. Many chargers
incorporate a "low temperature inhibit" to nullify this event.
3.8.6.3 Pack Voltage Measurement (PVM)
The measurement of the battery pack voltage is also part of the trickle charge step. A pack
voltage measurement (PVM) can be used to verify that the battery is at the proper voltage level
and to verify there is current available for charging. Time (few seconds to 10 minutes) and
voltage (1.1 volts x # of cells) are dependent on the type and number of cells used. If the
voltage of the battery is not reached in the set time (usually about 20 minutes) the charge is
terminated. For PVM a pulse charge at a rate of C/10 to C/50 is recommended, but a constant
charge rate of C/10 to C/50 can be used.
3.8.6.4 Rapid/Fast Charge
Rapid charge or fast charge methods require three modes of termination:
1. PVD, or –∆V, or dT/dt, or ∆T
2. Timer
3. Temperature Cut Off (TCO).
See Table 3.8.5 Charge Method Specification for charge rate information.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
3.8.6.5 Top-Off Charge
Top-off charging is only used if the fast charge or rapid charge does not fully charge the
batteries. This occurs with some dT/dt and ∆T terminations. Before using a dT/dt or ∆T fast
charging, the charging and termination parameters need to be tested inside the device. Since a
constant current top-off charge has a tendency to remove energy, the top-off charge is best to
be done as a pulse charge. The top off charge is terminated by time and is C/10 to C/40 of the
rapid charge rate.
3.8.6.6 Maintenance
A maintenance charge will retain a full charge on the battery until it is removed from the
charger. During the first 24 hours after a battery has been charged, it will lose about 5% of its
energy caused by the battery’s self-discharge. A standard maintenance charge at a C/128 rate
is designed to counter this self-discharge. Some NiMH chemistries are able to handle
maintenance charge rates up to a C/64 rate. Continuous charging at low rates is not very
efficient, therefore, pulse charging is recommended.
3.9
NiMH Battery Storage Overview
Over time capacity and voltage of NiMH rechargeable batteries will decrease when stored or left unused. This is
caused by a chemical reaction that takes place within the cells, commonly referred as self-discharge. The
affects of self-discharge will be minimized if unused batteries are properly stored. Proper storage of NiMH
batteries requires both temperature control and inventory management.
3.9.1 NiMH Storage Temperature and Battery Cycling
Temperature is the major factor affecting the rate of self-discharge of an unused battery. As storage
temperatures increase, the self-discharge rate increases thus causing the maximum storage time of a
battery to decrease. It is best to store batteries in a temperature-controlled environment so that the
maximum storage time can be accurately determined. Figure 3.9.1 Storage Temperature vs. Storage
Time shows the range of storage temperatures that NiMH batteries can be stored and the maximum
length of time a battery can be left unused before having to be cycled.
Figure 3.9.1 Storage Temperature vs. Storage Time
Storage
Temperature
Maximum Storage Time
(Frequency of Cycling)
40°C to 50°C (104°F to 122°F)
30°C to 40°C (86°F to 104°F)
-20°C to 30°C (-4°F to 86°F)
Less than 30 days
30 to 90 days
180 to 360 days
The energy storage capability of a battery will be decreased if the battery is allowed to completely selfdischarge. The affects of self-discharge can be corrected if the batteries are subjected to cycles of
charging and discharging. On the initial charge/discharge cycle, the battery will achieve approximately
95% of rated capacity. Full capacity will be achieved on the second and third cycles.
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Nickel Metal Hydride
Harding Battery Handbook For Quest® Rechargeable Cells and Battery Packs
Section 3
3.9.2 NiMH State of Charge
The state of charge (SOC) of an unused NiMH battery has no influence on the required storage
temperature or the maximum storage time (see Figure 3.9.1 Storage Temperature vs. Storage Time). A
fully discharge battery will last in storage as long as a fully charged battery. Therefore, NiMH batteries
can be stored at any state of charge.
3.9.3 NiMH Storage Guidelines
The key to properly storing NiMH rechargeable batteries is establishing good inventory management
practices. To prolong the cycle life and maintain battery performance adhere to the following five
guidelines:
1. Practice FIFO (First In First Out) inventory rotation.
2. Never store batteries under load.
3. Store batteries in a temperature-controlled environment (see Figure 3.9.1 Storage Temperature vs.
Storage Time).
4. Cycle batteries (see Figure 3.9.1 Storage Temperature vs. Storage Time).
5. Store batteries at 65% (±20%) humidity.
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Page 26